Along with endoscopes for medical and non-medical technical applications, whose viewing angle is parallel to the longitudinal axis of the endoscope shaft, endoscopes with other fixed viewing angles were developed from an early date. Here and throughout the present document, the viewing angle of an endoscope is always understood to mean the direction looking outward from the distal end of the endoscope, that is, the direction in which an object is situated that appears in the center of the image recorded by means of the endoscope. With many applications, however, a fixed viewing angle is a disadvantage. In the most unfortunate case, the endoscope must be replaced repeatedly during a medical procedure. In such cases it is advantageous to use an endoscope with a viewing angle that can be adjusted and/or modified in situ.
Observing an object in a cavity by means of an endoscope assumes as a rule that there is an illumination of the object. For this purpose an endoscope comprises, for example, lightwave conductors, in particular fiber optic cables, by means of which illuminating light is transmitted from the proximal end of the endoscope along the shaft to the distal end of the endoscope. Light outlet surfaces of the lightwave conductors on the distal end of the endoscope are positioned and configured in such a way that the entire field of vision or visual field is sufficiently illuminated.
In an endoscope with adjustable viewing angle, the illuminating light at the distal end of the endoscope is distributed in the simplest case in such a way that the entire visual field is illuminated, independently of the particular viewing angle selected. However, this can lead to a series of disadvantages. In particular, light capacity is wasted because, independently of the viewing angle that is in fact selected, all visual fields of all viewing angles are constantly selected. Thus, for a predetermined desired brightness, it is necessary to have a markedly higher light capacity available altogether than is needed with an endoscope with fixed viewing angle.
Another disadvantage derives from the fact that high-intensity illuminating light can photothermally or photochemically damage tissues or other objects. In an endoscope with fixed viewing angle, as a rule there is clearly too small a distance from the distal end of the endoscope to an object, at least in observing the acquired image. When a video camera is used on the endoscope, an automatic warning of users is also possible if the brightness of a captured image exceeds a predetermined threshold. However, in an endoscope with adjustable viewing angle, part of the illuminating light falls on objects lying outside the visual field. This avoids an undesired proximity of the distal end of the endoscope to these objects, and a resulting exposure of these objects to an excessive radiant power.
Another disadvantage is the fact that illuminating light that is at first radiated outside the visual field can be dispersed or reflected by objects or opaque media. The reflected or dispersed illuminating light can arrive directly or indirectly in the observation beam path. Consequently, contrasts and, especially in dark image areas, the distinguishability of objects can be reduced.
Yet another disadvantage arises from the fact that the degree of illumination or intensity of the illuminating light is essentially constant in the direction in which the viewing angle can be varied (often also called the vertical direction), while in the direction perpendicular thereto (often also called the horizontal direction), as a rule it decreases slightly toward the edge of the visual field. However, from endoscopes with fixed viewing angle, users as a rule are accustomed to a degree of illumination that slightly decreases toward the edge of the visual field both in the horizontal and in the vertical direction. A constant degree of illumination in the vertical direction can therefore be experienced as irritating.
Patent DE 600 15 375 T2 describes an arrangement of several prisms. One of the prisms can rotate around an axis in order to cast illuminating light at an adjustable viewing angle. The prisms, however, take up space that is no longer available with constantly smaller shaft cross-sections.